Move registry package into handler package

The goal is to free up the distribution/registry package to include common
registry types. This moves the webapp definitions out of the way to allow for
this change in the future.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/handlers/helpers.go

@@ -0,0 +1,32 @@
+package handlers
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+)
+
+// serveJSON marshals v and sets the content-type header to
+// 'application/json'. If a different status code is required, call
+// ResponseWriter.WriteHeader before this function.
+func serveJSON(w http.ResponseWriter, v interface{}) error {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	enc := json.NewEncoder(w)
+
+	if err := enc.Encode(v); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// closeResources closes all the provided resources after running the target
+// handler.
+func closeResources(handler http.Handler, closers ...io.Closer) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		for _, closer := range closers {
+			defer closer.Close()
+		}
+		handler.ServeHTTP(w, r)
+	})
+}